Re: [PATCH 3/3] migration/postcopy: handle POSTCOPY_INCOMING_RUNNING corner case properly

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wei Yang (richardw.yang@linux.intel.com) wrote:
> On Wed, Oct 09, 2019 at 01:36:34PM +0800, Peter Xu wrote:
> >On Wed, Oct 09, 2019 at 01:07:56PM +0800, Wei Yang wrote:
> >> On Wed, Oct 09, 2019 at 12:12:25PM +0800, Peter Xu wrote:
> >> >On Wed, Oct 09, 2019 at 09:02:04AM +0800, Wei Yang wrote:
> >> >> On Tue, Oct 08, 2019 at 05:40:46PM +0100, Dr. David Alan Gilbert wrote:
> >> >> >* Wei Yang (richardw.yang@linux.intel.com) wrote:
> >> >> >> Currently, we set PostcopyState blindly to RUNNING, even we found the
> >> >> >> previous state is not LISTENING. This will lead to a corner case.
> >> >> >> 
> >> >> >> First let's look at the code flow:
> >> >> >> 
> >> >> >> qemu_loadvm_state_main()
> >> >> >>     ret = loadvm_process_command()
> >> >> >>         loadvm_postcopy_handle_run()
> >> >> >>             return -1;
> >> >> >>     if (ret < 0) {
> >> >> >>         if (postcopy_state_get() == POSTCOPY_INCOMING_RUNNING)
> >> >> >>             ...
> >> >> >>     }
> >> >> >> 
> >> >> >> From above snippet, the corner case is loadvm_postcopy_handle_run()
> >> >> >> always sets state to RUNNING. And then it checks the previous state. If
> >> >> >> the previous state is not LISTENING, it will return -1. But at this
> >> >> >> moment, PostcopyState is already been set to RUNNING.
> >> >> >> 
> >> >> >> Then ret is checked in qemu_loadvm_state_main(), when it is -1
> >> >> >> PostcopyState is checked. Current logic would pause postcopy and retry
> >> >> >> if PostcopyState is RUNNING. This is not what we expect, because
> >> >> >> postcopy is not active yet.
> >> >> >> 
> >> >> >> This patch makes sure state is set to RUNNING only previous state is
> >> >> >> LISTENING by introducing an old_state parameter in postcopy_state_set().
> >> >> >> New state only would be set when current state equals to old_state.
> >> >> >> 
> >> >> >> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> >> >> >
> >> >> >OK, it's a shame to use a pointer there, but it works.
> >> >> 
> >> >> You mean second parameter of postcopy_state_set()?
> >> >> 
> >> >> I don't have a better idea. Or we introduce a new state
> >> >> POSTCOPY_INCOMING_NOCHECK. Do you feel better with this?
> >> >
> >> >Maybe simply fix loadvm_postcopy_handle_run() to set the state after
> >> >the POSTCOPY_INCOMING_LISTENING check?
> >> >
> >> 
> >> Set state back to ps if ps is not POSTCOPY_INCOMING_LISTENING?
> >> 
> >> Sounds like another option.
> >
> >Even simpler?
> >
> >  ps = postcopy_state_get();
> >  if (ps != INCOMING)

  ^^ LISTENING

> >    return -1;
> >  postcopy_state_set(RUNNING);
> >
> 
> Looks good to me.
> 
> Dave,
> 
> Do you feel good with it?

Yes, I think so; it's simpler.

Dave

> >Thanks,
> >
> >-- 
> >Peter Xu
> 
> -- 
> Wei Yang
> Help you, Help me
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK